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Surface Haptics Thermal Conductivity – Testing the Quality of Comfort

Surface Haptics is a branch of psychology, describing the sense of touch localized to the skin surface. These senses make up the feeling of comfort or the desired effect which goes into the touched material surface. This is an important field of measurement for industries like textiles and automotive interiors. Thermal effusivity is often used as a measurement for the sense of heat, as it is the property which governs the transfer of heat from one body to the next, when each body is at a different temperature.

The amount of temperature we feel using our sense of touch on different materials is a measurement of quality and comfort. The heat transfer from your hand, which acts like a temperature sensor when it comes in contact with a material at room temperature, will feel different depending upon the thermal effusivity of that material. Insulating materials such as foams, leather, and woods will feel warm to the touch when compared to conducting materials (metals, polymers, and carbon) which will feel cool. Materials with lower thermal effusivity will feel warm on our hands faster than a high thermal effusivity material. In surface haptics, the measurement of thermal effusivity becomes an important parameter.

Figure 1. Comparative diagram of wood vs. metal to the touch.

Using the transient plane source (TPS) technique for thermal properties measurements, thermal effusivity, thermal conductivity, and thermal diffusivity are directly measured without the need for calibration. This is done using the Hot Disk single-sided sensor which offers excellent accuracy and repeatability without the requirement of two pieces of sample. This is a very convenient method for testing large hard to handle samples.

Similar to their sister transient plane two-sided sensor, single-sided sensors only penetrate on one side. The ability to keep penetration in the desired direction is achieved with the help of backing insulation foam. Testing can still be performed on isotropic and anisotropic samples both through plane and in-plane of the material.

Figure 2. Hot Disk TPS single-sided sensor.

Thermal effusivity, also known as the heat penetration coefficient, is the rate at which a specific material can absorb heat. It is the property that determines the contact temperature of two bodies that touch each other. By definition, it is the product of the square root of thermal conductivity x density x specific heat in the units of W/cm2/k/s0.5

e=√(k∙ρ∙Cρ )

In a number of industrial applications, such as textiles and automotive interiors, the thermal effusivity is a critical design property to achieve the desired feeling. One measurement application would be the design of materials to look at feel like the interior of a luxury car. By measuring the thermal effusivity, which simulates the hand in terms of temperature, the desired comfort and quality feeling can be accomplished.

Figure 3. Overview of possible measurement locations.

Measurements of thermal effusivity can be taken from the leather seats (E), wood paneling on the dash and side door (C), the stirring wheel (A), the metal between the seats (B), and the polymer coating on the door (D).

As an experimental project, one of Thermtest’s scientists set out to measure the thermal effusivity of various materials inside a motor vehicle with the same layout as Figure 3. Even though the results were not relative to reference material due to large amounts of drift, slight movement of sensors, minimal penetration into material, and surface contact we were still able to conclude that the results follow the proper trending.

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